1. Field of the Invention
The present invention relates to a thin-film transistor of a liquid crystal display and, more particularly, to a thin-film transistor having a gate including a double-layered metal structure and a method of making such a double-layered metal gate.
2. Discussion of Related Art
An LCD (Liquid Crystal Display) includes a switching device as a driving element, and a pixel-arranged matrix structure having transparent or light-reflecting pixel electrodes as its basic units. The switching device is a thin-film transistor having gate, source and drain regions.
The gate of the thin-film transistor is made of aluminum to reduce its wiring resistance, but an aluminum gate may cause defects such as hillock.
A double-layered metal gate, i.e., molybdenum-coated aluminum gate is considered as a substitute for the aluminum gate to overcome the problem of the hillock.
To fabricate a double-layered gate, metals such as aluminum and molybdenum are sequentially deposited, followed by a patterning process carried out via photolithography to form resulting metal films which have the same width. Although the double-layered gate is desirable to overcome the problem of hillock, the resulting deposited metal films forming the double-layered gate are so thick that a severe single step is created by a thickness difference between the metal films and a substrate, thereby causing a single step difference between the substrate and the double-layered gate which deteriorates the step coverage of a later formed gate oxide layer. The source and drain regions formed on the gate oxide layer may have disconnections between areas of the source and drain regions which are overlapped and non-overlapped with the gate, or electrically exhibit short circuits as a result of contact with the gate.
According to another method of forming the gate, each of the metal layers of Al and Mo form a double step difference with the substrate so as to improve the step coverage of the gate oxide layer.
FIGS. 1A through 1F are diagrams illustrating the process for fabricating a thin-film transistor of a method which is related to the invention described and claimed in the present application. The method shown in FIGS. 1A-1F is not believed to be published prior art but is merely a recently discovered method related to the invention described and claimed in the present application.
Referring to FIG. 1A, aluminum is deposited on a substrate 11 to form a first metal layer 13. A first photoresist 15 is deposited on the first metal layer 13. The first photoresist 15 is exposed and developed so as to have a certain width w1 extending along the first metal layer 13.
Referring to FIG. 1B, the first metal layer 13 is patterned via wet etching using the first photoresist 15 as a mask so that the first metal layer 13 has a certain width w1. After the first photoresist 15 is removed, a second metal layer 17 is formed by depositing Mo, Ta, or Co on the substrate 11 so as to cover the first metal layer 13. A second photoresist 19 is then deposited on the second metal layer 17. The second photoresist 19 is exposed and developed so as to have a certain width w2 extending along the second metal layer 17 and located above the first metal layer 13.
Referring to FIG. 1C, the second metal layer 17 is patterned via a wet etching process using the second photoresist 19 as a mask such that the second metal layer 17 has a certain width w2 which is narrower than the width w1 of the first metal layer 13. After formation of the gate 21, the second photoresist 19 is removed.
Thus, the patterned first and second metal layers 13 and 17 form a gate 21 having a double-layered metal structure that provides a double step difference between the double-layered metal gate structure 21 and the substrate 11. The formation of the gate 21 as described above and shown in FIGS. 1-3 requires the use of two photoresists 15, 19 and two photoresist steps.
In the gate 21 shown in FIG. 3, the second metal layer 17 is preferably centrally located on the first metal layer 13. Although there is no specific information available regarding a relationship of w1 to w2 of this related method, based on their understanding of this related method resulting in the structure shown in FIG. 3, the inventors of the invention described and claimed in the present application assume that the width difference w1-w2 between the first and second metal layers 13 and 17 is larger than or equal to 4 .mu.m, that is, w1-w2.gtoreq.4 .mu.m.
Referring to FIG. 1D, a first insulating layer 23 is formed by depositing silicon oxide SiO.sub.2 or silicon nitride Si.sub.3 N.sub.4 as a single-layered or double-layered structure on the gate 21 and substrate 11. Semiconductor and ohmic contact layers 25 and 27 are formed by sequentially depositing undoped polycrystalline silicon and heavily doped silicon on the first insulating layer 23. The semiconductor and ohmic contact layers 25 and 27 are patterned to expose the first insulating layer 23 by photolithography.
Referring to FIG. 1E, conductive metal such as aluminum is laminated on the insulating and ohmic contact layers 23 and 27. The conductive metal is patterned by photolithography so as to form a source electrode 29 and a drain electrode 31. A portion of the ohmic contact layer 27 exposed between the source and drain electrodes 29 and 31 is etched by using the source and drain electrodes 29 and 31 as masks.
Referring to FIG. 1F, silicon oxide or silicon nitride is deposited on the entire surface of the structure to form a second insulating layer 33. The second insulating layer 33 is etched to expose a designated portion of the drain electrode 31, thus forming a contact hole 35. By depositing transparent and conductive material on the second insulating layer 33 and patterning it via photolithography, a pixel electrode 37 is formed so as to be electrically connected to the drain electrode 31 through the contact hole 35.
According to the method of fabricating a thin-film transistor as described above and shown in FIGS. 1A-1F, respective first and second metal layers are formed through photolithography using different masks so as to form the gate with a double-layered metal structure, resulting in double step differences between the gate and substrate.
As a result of the double step difference between the gate 21 and the substrate 11 shown in FIG. 1C, a hillock often occurs on both side portions of the first metal layer 13 which have no portion of the second metal layer 17 deposited thereon when the first metal layer 13 is wider than the second metal layer 17 as in FIG. 1C. Another problem with this related method is that the process for forming a gate is complex and requires two photoresists 15, 19 and two steps of deposition and photolithography. As a result, the contact resistance between the first and second metal layers may be increased.
Another method of forming a double metal layer gate structure is described in "Low Cost, High Quality TFT-LCD Process", SOCIETY FOR INFORMATION DISPLAY EURO DISPLAY 96, Proceedings of the 16th International Display Research Conference, Birmingham, England, Oct. 1, 1996, pages 591-594. On page 592 of this publication, a method of forming a double metal gate structure includes the process of depositing two metal layers first and then patterning the two metal layers to thereby eliminate an additional photoresist step. However, with this method, process difficulties during the one step photoresist process for forming the double metal layer gate resulted in the top layer being wider than the bottom layer causing an overhang condition in which the top layer overhangs the bottom layer. This difficulty may result in poor step coverage and disconnection. This problem was solved by using a three-step etching process in which the photoresist had to be baked before each of the three etching steps to avoid lift-off or removal of the photoresist during etching. This three-step etching process and required baking of the photoresist significantly increases the complexity and steps of the gate forming method.